Speed and frequency of information communication equipment have recently been increased, and hence electronic circuits including a passive element used in it have increased in complexity. Therefore, in designing the electronic circuits, a highly accurate circuit simulation in a time region of the electronic circuits, namely a transient response analysis, is important.
Electric characteristics of these complex electronic circuits are difficult to be predicted in the designing stage of a circuit diagram. The following process is therefore, generally performed. A prototype of an electronic circuit is produced, its electric characteristic is measured, and, when the electric characteristic is not a desired one, the electronic circuit is redesigned. For speeding up this process, a circuit simulation for predicting the electric characteristic of the electronic circuit has been performed using a computer and software.
A typical example of the software is SPICE (Simulation Program with Integrated Circuit Emphasis) developed in University of California. The SPICE simulates the electric characteristic using an equivalent circuit model of the electronic circuit which is derived by reproducing a transfer function in a frequency region. This method has high accuracy, but the derivation of the equivalent circuit model is disadvantageously complicated.
While, in a conventional method of the transient response analysis using no equivalent circuit model, the transfer function is inversely Fourier-transformed to provide an impulse response, and a response signal is determined by convolution of the impulse response and an input signal.
FIG. 7 shows a procedure of this conventional method. A capacitor is used as the passive element, and impedance is used as the transfer function.
Firstly, impedance of a frequency region is provided, namely is input, in first step 71. This impedance is a value measured by a measuring apparatus such as an inductance-capacitance-resistance (LCR) meter.
Then, the input impedance is inversely Fourier-transformed to provide an impulse response in second step 72. The impedance is transformed from a value in the frequency region to a value in a time region.
A current value in the time region is then provided as an input signal in third step 73. Namely, an appropriate current value is input.
The impulse response and the current value are then convoluted to provide a voltage value as a response signal in fourth step 74.
The transient response analysis is performed in these steps.
This method can advantageously omit labor of the derivation of the equivalent circuit model. However, the analyzing accuracy depends on the accuracy of the transfer function and the size of the concerned frequency region, so that sufficient accuracy cannot be obtained when a capacitor is used as the passive element.
The absolute value of the imaginary part of the impedance of the capacitor disperses inversely proportionally to frequency, so that it is difficult to measure an impedance at zero frequency that has a large effect on an analysis result. Therefore, sufficient analyzing accuracy cannot be obtained.